Joining and Surface Modification of Age-Hardening Alloys through Laser Processing

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In the present research, laser processes have been applied to joining and surface modification of age-hardening alloys and their beneficial characteristics have been developed.In the laser beam welding of age hardening 6061-T6 aluminum alloy and Ni base super alloy, Inconel 718, the widths of softened regions were extremely narrower than those of tungsten inert gas (TIG) welds.The softening comes from the dissolution of age precipitates, beta" in 6061-T6, and gamma' and gamma"in Inconel 718, by a weld heat input.Thus, a low overall heat input of laser beam welding can minimize the softened regions.The laser beam weld joints had higher strengths than those of TIG weld joints after post-weld aging heat treatments for both alloys.This advantageous characteristic of laser beam welding was quantitatively evaluated through predicting hardness distributions in weld heat affected zones using a newly developed kinetic equation describing the dissolution of age precipitates.Prediction of the width of softened regions with varying power inputs and welding speeds reveals that a high energy density and a high welding speed in laser beam welding results in significantly narrower softened regions, in which the width is insensitive to variations in welding parameters compared to that of TIG welding.Laser processing was also applied to surface modification of Inconel 718 for reducing sensitivity to hydrogen embrittlement of the alloy.Scanning laser beam on to the surface of Inconel 718 could locally produce surface solutionized zones.The surface solutionized zones were softened by the dissolution of age precipitates.This laser surface solutionizing is effective in preventing hydrogen cracking of Inconel 718 with little sacrifice of the strength of structures.The ductility of the surface solutionized specimen was almost twice that of the base metal in a tensile test under a 29.4MPa hydrogen atmosphere at room temperature with hydrogen pre-charging.